The purpose of this project is to define the pattern and sequence of development of the adrenal rhythm as defined by changes in serum corticosterone levels in young rats and to determine (1) the role of different neural inputs to the hypothalamus on the origin and maintenance of the rhythm (2) to compare the developmnt of other rhythmicities (sleep-wakefulness, body temperature and food intake) with the patterns worked out for the adrenal rhythm and to determine whether there are critical periods during the development of rhythmicity when environmental manipulations will have long-range influences on the expression of rhythms and upon puberty. Specific goals for this year include (1) further definition of the role of environmental photoperiod in conditioning the development or maintenance of the adrenal rhythm and the body temperature rhythm (2) a study of the role of gonadal status in rhythm maturation and maintenance (3) an exploration of the role of the suprachiasmatic nuclei in rhythm onset, maintenance and resetting by studying the response of rats with hypothalamic transections or suprachiasmatic lesions to changes in photoperiod.